Xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomer is para-xylene, the principal feedstock for polyester, which continues to enjoy a high growth rate from large base demand. Ortho-xylene is used to produce phthalic anhydride, which supplies high-volume but relatively mature markets. Meta-xylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but is usually considered a less-desirable component of C8 aromatic hydrocarbons (the notation Cx in the present disclosure is used to indicate hydrocarbon compounds having “x” carbon (C) atoms, as is conventional in the art).
Among the aromatic hydrocarbons, the overall importance of xylenes rivals that of benzene as a feedstock for industrial chemicals. Xylenes and benzene are produced from petroleum by reforming naphtha but not in sufficient volume to meet demand, thus conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Often toluene and/or C9+ aromatic hydrocarbons are de-alkylated to produce benzene or selectively disproportionated to yield benzene and C8 aromatic hydrocarbons from which the individual xylene isomers are recovered.
C8 aromatic hydrocarbons that are derived from hydrotreated and extracted pyrolysis gasoline (“pygas”) have too great of an ethylbenzene content to be simply fractionated and sold as mixed xylenes (as used herein, the term “mixed xylenes” refers to a mixture of each of the three xylene isomers (ortho-, meta-, and para-) and ethylbenzene, which may or may not be in an equilibrium ratio). In the prior art, ethylbenzene has been super-fractionated from xylenes. However, this requires many fractionation trays (for example, possibly greater than 300) and high reflux rates (for example, possibly greater than 10 reflux vol./feed vol. (R/F)) to enable effective separation between ethylbenzene and para-xylene, which have only a 2° C. boiling point difference. Other aromatic hydrocarbon products from pygas include benzene, toluene, and C9+ aromatic hydrocarbons, which must be accounted for and separated as well. While crude ethylbenzene can be valuable as a gasoline blending stock, as noted above, it is more valuable if it can be converted into a benzene product and also possibly an ethane product (which can be recovered as feed for a stream cracker).
Accordingly, it is desirable to provide improved methods and apparatuses for processing aromatic hydrocarbon feedstocks that include too much ethylbenzene for commercial sale. Furthermore, it is desirable to provide such methods and apparatuses that are capable of converting the ethylbenzene in such feedstocks to more valuable products, such as benzene, xylenes, and possibly also ethane. Still further, it is desirable to provide such methods and apparatuses that do not require the use of costly super-fractionation processes. Furthermore, other desirable features and characteristics of the presently disclosed embodiments will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.